The present invention relates generally to a flexible drill string system for drilling long holes in a mine roof, and more particularly to such a system wherein thrust and torque are supplied to a straightened portion of the drill string at the working surface of the roof to maximize torque and thrust components transmitted to the drill head.
High grade coal is generally mined underground in narrow seams of the coal between layers of sedimentary rock (usually shale or sandstone). The shale roof of the mine is commonly supported by a process known as roof bolting. This process involves drilling a hole (typically 13/8 inch in diameter and 4-6 feet long) vertically into the shale. A nut is wedged tightly against the shale at the bottom of the hole into stable rock. Then a rock bolt having a length about equal to the length of the hole is inserted, threaded into the nut, and torqued down tightly against the roof. Thus, the unstable rock at the roof is clamped to the more stable rock above it.
A problem arises where a standard 72 inch long bolt hole has to be drilled into the roof of the mine, and the coal seam is only, e.g., 40 inches high. Obviously, a one-piece drill shaft is too long because it cannot be oriented perpendicular to the roof.
If the one-piece drill shaft is replaced by a flexible shaft formed of a series of interconnected drill segments, a problem arises in transmitting thrust to the drill head along the string for advancing the head into the rock. Since the drill string is perpendicular to the roof at the working surface, and extends downwardly and away from the working surface along a curve to fit within the limited height of the seam, torque and thrust supplied to the string at the end opposite the drill head is transmitted to the head along the curved portion of the drill string. Thus, only a fractional component of thrust reaches the head with the remainder being taken up in the curved portion of the drill string. Furthermore, since thrust and torque are transmitted to the head of the drill along all of the drill segments, excessive wear of the coupling members between segments is created, and this results in increased breakage and down time.